where in just a few hours, a sleeping spacecraft will wake up from a 2 1/2 year nap. It's called the Rosetta and it was actually launched ten years ago on an extraordinary mission to land on a comet. After a 500 million mile journey, will it be mission accomplished? Here's ABC's lama Hasan. Reporter: It's a space mission like no other. Launched a decade ago, the Rosetta satellite has been in hibernation for the last three years to conserve power. But if all goes well, early tomorrow an internal alarm clock will go off, and Rosetta will awaken to begin the next part of the odyssey. Chasing down a comet racing towards the sun at 24,000 miles an hour. Something scientists compare to landing on a speeding bullet. When it catches up in August, Rosetta will have to fire harpoons into its icy surface to complete the first ever landing on a comet. Scientists theorize that comets might have delivered the first water to Earth. Perhaps even seeding the planet with the building blocks of life. Ultimately, the hope is Rosetta will help us understand and unlock some of the most important secrets of our solar system.

This transcript has been automatically generated and may not be 100% accurate.

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